1. Field of the Invention
This invention relates to a new process for the preparation of mixed trimers from organic isocyanates of a reactivity which differs insofar as the isocyanate trimerization reaction is concerned, to the mixed trimers obtained according to the process and to the use of the mixed trimers obtained according to the process, containing free isocyanate groups, optionally in a form blocked by blocking agents for isocyanate groups, for the production of polyurethane plastics.
2. Description of the Prior Art
Processes for trimerizing organic isocyanates are known (see, for example, B. A. Farkas, G. A. Mills, Advances in Catalysis, Vol. 13, 393 (1962); J. H. Saunders, K. C. Frisch, Polyurethanes, Chemistry and Technology, Interscience Publishers, New York (1962); J. E. Kresta, K. H. Hsieh, Org. Coat, Plast. Chem., 42, 516 (1980); German Offenlegungsschrift Nos. 1,201,992; 1,954,093; 2,325,826; 2,551,634; 2,641,380; 2,644,684; 2,726,749 and 2,901,479; British Pat. No. 949,253; Published European Patent Application Nos. 9694 and 10589; U.S. Pat. No. 3,622,577 or German Patent Application Nos. 3,100,262.5 or 3,100,263.3).
The preparation of mixed trimers from aromatic and aliphatic diisocyanates is described, for example, in German Pat. No. 1,670,667. For this purpose, mixtures of hexamethylene diisocyanate and 2,4-diisocyanatotoluene, for example, are partly trimerized in the presence of organic phosphines as the trimerization catalyst, during which operation, however, the aliphatic diisocyanate is incorporated into the trimer to a much smaller extent than that which corresponds to its concentration in the starting mixture. Thus, the aliphatic isocyanates are used simultaneously in the process of this prior publication not so much with the objective of preparing mixed trimers, but rather due to the cocatalytic influence of the aliphatic diisocyanate on the phosphine-catalyzed trimerization of aromatic diisocyanates. In spite of this co-catalytic effect of the aliphatic diisocyanate, considerable quantities of non-trimerized aromatic starting diisocyanates are still always present in the reaction mixtures described by the prior publication after terminating the trimerization reaction so that while recovering the unreacted diisocyanates their separation into the aliphatic and aromatic starting diisocyanates is necessary. Another disadvantage of the process described in German Pat. No. 1,670,667 is seen in the essential use of organic phosphines as trimerization catalysts. Since organic phosphines are primarily dimerization catalysts for organic diisocyanates, the trimerization reaction in the process of German Pat. No. 1,670,667 presumably also takes place via the intermediate stage of dimers (uretdiones). Moreover, the formation of mixed trimers in the process of the prior publication can only be explained based on this specific reaction mechanism. During the trimerization of mixtures of isocyanates of different reactivity with other trimerization catalysts which do not form uretdiones, homotrimers of the aromatic diisocyanates are mostly produced because the aliphatic diisocyanates which are slower to react only participate in the trimerization reaction after a certain degree of dilution of the aromatic diisocyanates is reached. Accordingly, mixed trimers only form to a slight extent while aromatic and aliphatic homotrimers are preferably formed. Pure mixed trimers are only produced during the trimerization of isocyanate mixtures when mixtures of isocyanates having essentially chemically equivalent NCO groups are trimerized. Thus, for example, the two aromatic monoisocyanates, 4-chlorophenylisocyanate and 3,4-dichlorophenylisocyanate, may be mix-trimerized with suitable catalysts, the proportion of mixed trimer being determined by the composition of the mixture.
In European Polymer Journal, Vol. 17, page 35 (1981), S. Dabi and A. Zilkha have recently described a specific type of catalyst, tributyl tin oxide, for the mixed trimerization of less frequently used mono- and diisocyanates; however, the process which is recommended requires temperatures of above 100.degree. C. and a proportion of catalyst greater than 1% by weight, which greatly impairs the quality of the resulting products and rules out a commercial use, in particular, for the polyurethane lacquer area. Moreover, uretdiones are also formed as by-products with this type of catalyst.
The object of the present invention is to provide a general preparation process, according to which pure mixed trimers are obtained as the main product in one step from any mixtures of isocyanates of differing reactivity using a catalyst. The process conditions should be similar to those of the prior art for the homo-trimerization of polyisocyanates into polyisocyanates containing isocyanurate groups because the resulting products are preferably used for the production of polyurethane plastics. However, a commercial possibility of using the mixed trimers containing isocyanate groups is only provided (1) if the proportion of catalyst is small enough that it may easily be deactivated and optionally separated, (2) if the thermal strain is so low that no disadvantageous discolorations of the products occur and (3) if no uretdiones are formed as by-products because the toxicologically harmful monomeric isocyanates could be produced therefrom during storage as a result of splitting back.
Surprisingly, it has now been found that this object may be achieved by the process according to the present invention which is described in more detail in the following, and in which (1) the isocyanate which is slower to react is introduced in excess, (2) is mixed with a quantity of catalyst sufficient for the trimerization thereof, (3) trimerization of the solution is started for a short time and (4) the more reactive isocyanate is then continuously added to the slower-reacting activated isocyanate. In the process according to the present invention, more than about 50% and preferably more than about 70% of pure mixed trimers are produced based on the total quantity of the resulting trimers, and the proportion of the isocyanates of lower or higher reactivity which are incorporated in the mixed trimer may be simply controlled by the rate of adding the isocyanate component of higher reactivity and by the total ratio of (isocyanate component of lower reactivity):(isocyanate component of higher reactivity).